The term "microwaves" refers to electromagnetic energy in frequencies covering the range of about 0.1 gigahertz (GHz) to 1,000 GHz with corresponding wavelengths from about 300 centimeters to about 0.3 millimeters. Although "microwaves" are perhaps most widely associated by the layperson with cooking devices, those persons familiar with electronic devices recognize that the microwave frequencies are used for a large variety of electronic purposes and in corresponding electronic devices, including various communication devices, and the associated circuit elements and circuits that operate them. As is the case with many other semiconductor electronic devices and resulting circuits, the ability of a device (or circuit) to exhibit certain desired or necessary performance characteristics depends to a large extent, and often entirely, upon the material from which it is made. One appropriate candidate material for microwave devices is silicon carbide, which offers a primary advantage for microwave applications of a very high electric breakdown field. This characteristic of silicon carbide enables devices such as metal semiconductor field effect transistors (MESFETs) to operate at drain voltages ten times higher than field effect transistors formed in gallium arsenide (GaAs).
Additionally, silicon carbide has the significant advantage of a thermal conductivity of 4.9 watts per degree Kelvin per centimeter (W/K-cm) which is 3.3 times higher than silicon and ten times higher than either gallium arsenide or sapphire. These properties give silicon carbide a high power density in terms of gate periphery measured in terms of watts per millimeter (W/mm) and also an extremely high power handling capability in terms of die area (W/mm). This is particularly advantageous for high power, high frequency applications because die size becomes limited by wavelength. Accordingly, because of the excellent thermal and electronic properties of silicon carbide, at any given frequency, silicon carbide MESFETs should be capable of at least five times the power of devices made from gallium arsenide.
As recognized by those familiar with microwave devices, they often require high resistivity ("semi-insulating") substrates for coupling purposes because conductive substrates tend to cause significant problems at microwave frequencies. As used herein, the terms "high resistivity" and "semi-insulating" can be considered synonymous for most purposes. In general, both terms describe a semiconductor material having a resistivity greater than about 1500 ohm-centimeters (.OMEGA.-cm).
Such microwave devices are particularly important for monolithic microwave integrated circuits (MMICs) which are widely used in communications devices such as pagers and cellular phones, and which generally require a high resistivity substrate. Accordingly, the following characteristics are desirable for microwave device substrates: A high crystalline quality suitable for highly complex, high performance circuit elements, good thermal conductivity, good electrical isolation between devices and to the substrate, low resistive loss characteristics, low cross-talk characteristics, and large wafer diameter.
Given silicon carbide's wide bandgap (3.2 eV in 4H silicon carbide at 300K), such semi-insulating characteristics should be theoretically possible. As one result, an appropriate high resistivity silicon carbide substrate would permit both power and passive devices to be placed on the same integrated circuit ("chip") thus decreasing the size of the device while increasing its efficiency and performance. Silicon carbide also provides other favorable qualities, including the capacity to operate at high temperatures without physical, chemical, or electrical breakdown.
As those familiar with silicon carbide are aware, however, silicon carbide grown by most techniques is generally too conductive for these purposes. In particular, the nominal or unintentional nitrogen concentration in silicon carbide tends to be high enough in sublimation grown crystals (1-2.times.10.sup.17 cm.sup.-3) to provide sufficient conductivity to prevent such silicon carbide from being used in microwave devices.
Some recent efforts have attempted to compensate the effective level of nitrogen by adding a p-type (i.e., acceptor) dopant such as boron. In practice, however, SiC-based devices made using boron to obtain high resistivity have exhibited unexpectedly poor results at high power levels. Additionally, in comparison to some other elements, boron tends to diffuse relatively well in SiC, giving it an undesirable tendency to migrate into adjacent device layers and unintentionally affect them.
In order to be particularly useful, silicon carbide devices should have a substrate resistivity of at least 1500 ohm-centimeters (.OMEGA.-cm) in order to achieve RF passive behavior. Furthermore, resistivities of 5000 .OMEGA.-cm or better are needed to minimize device transmission line losses to an acceptable level of 0.1 dB/cm or less. For device isolation and to minimize backgating effects, the resistivity of semi-insulating silicon carbide should approach a range of 50,000 .OMEGA.-cm or higher. Present work tends to assert that the semi-insulating behavior of a silicon carbide substrate is the result of energy levels deep within the band gap of the silicon carbide; i.e., farther from both the valence band and the conduction band than the energy levels created by p-type and n-type dopants. These "deep" energy levels are believed to consist of states lying at least 300 meV away from the conduction or valence band edges, e.g., U.S. Pat. No. 5,611,955 which is representative of current conventional thinking in this art. According to the '955 patent, the deep levels in the silicon carbide between the valence and conduction bands can be produced by the controlled introduction of selected elements such as transition metals or passivating elements such as hydrogen, chlorine or fluorine, or combinations of these elements into the silicon carbide to form the deep level centers in the silicon carbide; e.g., column 3, lines 37-53. See also, Mitchel, The 1.1 eV Deep Level in 4H-SiC. SIMC-X, Berkley Calif., June 1998; Hobgood, Semi-Insulating GH-SiC Grown by Physical Vapor Transport, Appl. Phys. Lett. Vol. 66, No. 11 (1995); WO 95/04171; Sriram, RF Performance of SiC MESFETs on High Resistivity Substrates, IEEE Electron Device Letters, Vol. 15, No. 11 (1994); Evwaraye, Examination of Electrical and Optical Properties of Vanadium in Bulk n-type Silicon Carbide, J. Appl. Phys. 76 (10) (1994); Schneider, Infrared Spectra and Electron Spin Resonance of Vanadium Deep Level Impurities in Silicon Carbide, Appl. Phys. Lett. 56(12) (1990); and Allen, Frequency and Power Performance of Microwave SiC FET's, Proceedings of International Conference on Silicon Carbide and Related Materials 1995, Institute of Physics.
Further to the conventional thinking, these deep level elemental impurities (also known as deep level trapping elements) can be incorporated by introducing them during high temperature sublimation or chemical vapor deposition (CVD) growth of high purity silicon carbide. In particular, vanadium is considered a desirable transition metal for this purpose. According to the '955 patent and similar art, the vanadium compensates the silicon carbide material and produces the high resistivity (i.e., semi-insulating) characteristics of silicon carbide.
The introduction of vanadium as a compensating element to produce semi-insulating silicon carbide, however, also introduces certain disadvantages. First, the presence of electronically significant amounts of any dopant, including vanadium, can negatively affect the crystalline quality of the resulting material. Accordingly, to the extent that vanadium or other elements can be significantly reduced or eliminated, the crystal quality of the resulting material, and its corresponding electronic quality, can be increased. In particular, the present understanding is that compensating amounts of vanadium can cause growth defects such as inclusions and micropipes in silicon carbide.
As a second disadvantage, the introduction of compensating amounts of vanadium can reduce the yield and add expense to the production of semi-insulating silicon carbide substrates. Third, the proactive compensation of silicon carbide, or any other semiconductor element, can be somewhat complex and unpredictable and thus introduces manufacturing complexity that can be desirably avoided if the compensation can be avoided.